1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image using a developing agent and more particularly to a color image forming apparatus for forming an image with developing agents of plural colors.
2. Description of the Related Art
In recent years, a variety of color image forming apparatuses capable of outputting color images, such as color copiers and color printers have been provided following demands the market.
For example, an image forming apparatus which transfers an image by using a semiconductive transfer belt and a transfer roller provided on the rear face of the transfer belt has been well known as disclosed in Jpn. Pat. Appln. KOKAI Publicaiton No. 6-110343. This publication has disclosed that an image is transferred by applying a transfer bias to the transfer roller.
In the color image forming apparatus which forms an image with plural toners, namely, yellow (Y), magenta (M), cyan (C) and black (Bk), as disclosed in the above publication, the following methods have been well known;
(1) A method of forming toner images of four colors on a photoconductor one over another and transferring an image composed of these plural toner images to a transfer object medium at once.
(2) A transfer drum method of forming images of four colors on a transfer object medium held on a transfer drum with four rotations of the drum, such that the toner images from the transfer drum rotating a single turn for each color are placed into layers.
(3) An intermediate transfer body method of forming a color image by placing toner images of four colors one over another on an intermediate transfer body and transferring that image to a transfer object medium at once.
(4) A consecutive four-drum method in which with four photoconductors disposed in parallel, images of four colors are formed to a transfer object medium moving in an opposing condition, corresponding to a rotation direction of each of four photoconductors rotating in the same direction, during each passing of the transfer object medium.
Because the color image forming apparatus which adopts the consecutive four-drum method can form a color image, which is transferred in multi-layers onto a transfer object medium while the transfer object medium passes a side opposing the four photoconductors, an image can be formed approximately in a quarter of time taken for the other methods 1 to 3 to proceed the four-color image forming process and thus, this method is suitable for an image forming apparatus for high-speed color image printing.
However, such a color image forming apparatus adopting the consecutive four-drum method carries a risk that part of a previously transferred toner image may be inversely transferred to a photoconductor in a process in which toner images of respective colors formed on the photoconductors are transferred to a transfer object medium or a transfer belt successively. That is, when a color image is formed, a toner adhering to the transfer object body or the transfer belt can be inversely transferred to a photoconductor of a different color when the toner image of a next color is formed. If this inverse transfer occurs, a developing agent transferred inversely invades into a photoconductor or a developing unit of a next color unless a photoconductor cleaner is provided, so that mixing of colors occurs. Due to this mixing of colors, the hue of a formed image changes thereby making it impossible to reproduce a color stably.
On the other hand, even an image forming apparatus equipped with the cleaner has such a problem that the quantity of adhering toner of each color toner image transferred onto the transfer object medium or the transfer belt is decreased by the inverse transfer. Because a transferred toner image of a first color passes three photoconductors until a toner image of a fourth color is transferred, reduction of the quantity of the adhering toner is more remarkable than other transferred toner images. Thus, this kind of color image forming apparatus has another problem that a difference in density occurs between toner images of respective colors, thereby lowering picture quality.